Electrophotographic photoreceptor is the key component contributing to the image formation in laser printers, copiers, and facsimile machines. In the electrophotographic printing process, image formation is accomplished by a sequence of related steps including: charging, exposure, developing, transfer, fixing, and erasure. All these steps are achieved by cooperative interactions among respective components that are centered around the electrophotographic photoreceptor. A photoreceptor exhibiting facile photoelectric response is extremely desirable for achieving good print quality. Recent developments in electrophotographic printing technology have allowed print resolutions of 600 dpi (dots per inch), and even 1200 dpi in some more advanced laser printers. The enhanced resolution is achieved via a laser modulation technique by modifying the length and amplitude of laser pulses. The associated imaging components in the print cartridge are required to be improved accordingly to match the fine resolution provided by the improved laser component. Toners with sufficiently reduced particle size have also been developed to generate very fine resolutions for printed images. In the delicate process of electrophotography, the photoreceptor receives latent images imparted by the laser beam which, thereafter, attracts toner particles and then transfers the attracted toner to a transfer medium such as paper or transparency. Therefore, photoreceptors exhibiting high photosensitivity and other desirable photoelectric properties are of paramount importance in order to achieve high resolution printing quality.
At the present time, most electrophotographic photoreceptors are made of organic electro-active materials due to the many advantages over inorganic materials in such areas as manufacturing cost, flexibility in structure configuration, non-toxicity, etc. In organic electrophotographic photoreceptors, a function-separated format is commonly utilized to provide photoreceptors with the abilities to both generate and transport charge carriers efficiently. The function-separated format comprising a charge transport layer on top of a charge generation layer not only facilitates the generation of free charge-carriers but also allows a wide variety of design options to be selected for choosing the optimal abrasion-resistant binder resin to be used in preparing the charge transport layer.
In the fabrication of photoreceptors with high photosensitivity for the use in high-resolution printers, the following design criteria have been developed:
(1) selecting charge generation materials with high charge generation efficiency; PA1 (2) selecting charge transport materials with expedient charge transport mobility; and PA1 (3) obtaining a good match between charge generation and charge transport materials to achieve negligible electric resistance at the interface formed therebetween.
Considerable research efforts have been focused on finding organic photoactive materials which can exhibit efficient charge generation upon exposure to light, so that they can be used as a charge generation material for use in electrophotographic photoreceptors. Typically, the charge generation materials for use in commercial applications must exhibit photosensitivity when exposing to irradiation between 750.about.850 nm in the case of laser printers using the semiconductor diode laser as the light source. Some well-known near-infrared sensitive organic materials include squarulenes, phthalocyanines and perylenes. Among them titanyl phthalocyanine is especially of interest due to its very high efficiency of charge generation. It has been shown in numerous prior art teachings that the charge generation efficiency of titanyl phthalocyanine (TiOPc) is very high and which can strongly depend on the crystal structure of the material. It was shown in U.S. Pat. No. 4, 898, 799 that highly sensitive Y-TiOPc can be obtained by the treatment of sulfuric acid and chlorine-containing solvents on the material. Other teachings such as U.S. Pat. Nos. 5,132,197 and 5,432,278 have shown the treatment of the water paste of the material with n-butyl ether can result in a high-sensitivity crystal form. A different technique employing the complexation reaction using ammonia gas as the crystal transformation medium was disclosed in U.S. Pat. No. 5,567,559 to obtain the highly sensitive titanyl phthalocyanine.
Well-known charge transport materials include organic molecules containing hydrazone, oxazole, pyrazoline, and triarylamine. Triarylamine molecules are the group of materials exhibiting very high hole drift mobility and receive much attention toward the aim of fabrication of high-sensitivity photoreceptors. The teachings of U.S. Pat. Nos. 4,081,274, 4,145,116 and 4,336,158 disclosed certain electroactive molecules which contain triarylamine moiety in the structure, can exhibit a very high hole mobility and result in high photosensitivity. More recently disclosed teachings such as U.S. Pat. Nos. 5,445,909 and 5,494,766 also disclosed high-mobility hole transport molecules that can also be categorized as triarylamine molecules.
However, these two approaches taught by the above-mentioned teachings are subject to limitation on the photosensitivity due to the limited extent of charge injection into the charge transport layer. In the two-layered configuration disclosed in U.S. Pat. Nos. 4,265,990, 4,233,384 and 4,306,008, it was believed that the two electrically operative layers including a charge generation layer and a charge transport layer are subject to an interface barrier for charge injection and the photosensitivity of the as-prepared photosensitive member is limited.
U.S. Pat. No. 5,476,740 disclosed a two-layered configuration in which the distinct interface between the charge generation layer and the charge transport layer is intentionally eliminated. The art utilized a technique to coat a charge transport layer on an undried charge generation layer to form a photosensitive member with a "merged" charge transport layer. It was claimed that by disrupting the well-defined interface, the charge generation material and the charge transporting material can be mixed more efficiently and therefore the otherwise formed interfacial barrier to the charge carrier injection is removed. However, the '740 patent has some inherent drawbacks when it is utilized in large scale production operations. In mass-fabrication processes, the undried charge generation material can diffuse into the charge transport layer and cause difficulties in operation.
Other prior art teachings such as U.S. Pat. Nos. 4,518,669, 4,579,801 and 5,391,448 disclosed a configuration which utilizes electrically conductive particles in the intermediate subbing layer, so as to obtain a smoothened inner layer underlying the charge generation layer and the charge transport layer. However, these prior art teachings were only aimed to suppress reflection of light from the substrate and to improve the electric grounding properties. They did not affect or improve the photosensitivity of the photosensitive member.